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Lecture 15 Explosive Nucleosynthesis and the R-Process

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Lecture 15 Explosive Nucleosynthesis and the R-Process Lecture 15 Explosive Nucleosynthesis and the r-Process As the shock wave passes through the star, matter is briefly heated to temperatures far above what it would have experienced in hydrostatic equilibrium. This material expands, then cools nearly adiabatically (if the energy input from shock heating exceeds that from nuclear burning). The time scale for the cooling is approximately the hydrodynamic time scale, though a little shorter. For (post-helium) burning in hydrostatic equilibrium, recall ε ≈ ε nuc ν hydrostatic nucleosynthesis advanced stages of stellar evolution For explosive nucleosynthesis : τ nuc (Tshock ) ≤ τ HD ρ ∝ T 3 ρ(t) = ρshock exp(−t / τ HD ) 9 −3/ 4 T(t) = Tshock exp(−t / 3τ HD ) Tshock ≈ 2.4 x10 R9 446 sec ρshock ≈ 2 ρo τ HD = ρ shock Except near the "mass cut", the shock temperature to which the explosive nucleosynthesis is most sensitive is given very well by 4 π R3 aT 4 ≈ Explosion energy 3 ≈ 1051 erg Example: Any carbon present inside of 109 cm will burn explosively if: 0.1 HD Conditions for explosive burning (E0 = 1.2 B): To nse T9 >5 R9 < 0.38 yes Silicon 4 − 5 0.38 − 0.51 yes Oxygen 3 − 4 0.51 − 0.74 yes Neon 2.5 −3 0.74−1.0 a little Carbon 2.0 − 2.5 1.0−1.3 no Helium >0.5 < 8 no Hydrogen >0.2 < 27 no −4/3 ⎛ T ⎞ Roughly speaking, everything that is R = 9 9 ⎜ 2.4 ⎟ ejected from inside 3800 km in the ⎝ ⎠ presupernova star will come out as iron-group elements. Ni O Si ESiB EOB ENeB Ejected without much modification Produced pre-explosively and just ejected in the supernova: • Helium • Carbon, nitrogen, oxygen • The s-process • Most species lighter than silicon Produced in the explosion: • Iron and most of the iron group elements – Ti, V, Cr, Mn, Fe Co, Ni • The r-process (?) • The neutrino process – F, B Produced both before and during the explosion: • The intermediate mass elements – Si, S, Ar, Ca • The p-process (in oxygen burning and explosive Ne burning) 25 Solar Masses; Rauscher et al. (2002) The amount of iron group synthesis (56Ni) will depend sensitively upon the density distribution around the collapsed core. Higher mass stars will synthesize more iron. The nucleosynthesis that results from explosive silicon burning is sensitive to the density (and time scale) of the explosion. 1) High density (or low entropy) NSE, and long time scale: Either the material is never photodisintegrated even partially to particles or else the -particles have time to reassemble into iron-group nuclei. The critical (slowest) reaction rate governing the reassembly is (2,)12C which occurs at a rate proportional to 2. If as T →0, Xn , X p , and Xα →0 then one gets pretty much the unmodified "normal" results of nuclear statistical equilibrium calculated e.g.,at T9 = 3 (fairly indedendent of ρ). Abundant at η= 0.002 − 0,004 56,57Ni, 55Co, 52,53,54Fe, 48,49,50Cr, 51 Mn Products: 54,56 55 48,49 50,51,52,53 51 Fe, Mn, Ti, Cr, V 2) Low density or rapid expansion à the “-rich” freeze out If all the α's cannot reassemble on τ HD , then the composition will be moditied at late times by α-capture. The composition will "freeze out" with free α-particles still present (and, in extreme cases, free n's or p's). The NSE composition at low T willbe modified by reactions like 54 Fe(α,γ )58 Ni 56 Ni(α, p)59 Cu 56 Ni(α,γ )60 Zn 40Ca(α,γ )44Ti 57 Ni(α,γ )61 Zn 58 Ni(α,γ )62 Zn etc. Abundant: 44Ti, 56,57,58Ni, 59Cu, 60,61,62Zn,(64,66Ge) Produced : 44Ca, 56,57Fe, 58,60,61,62 Ni, 59Co,(64,66Zn) 3) Explosive oxygen burning (3 ≤ T9 ≤ 4) Makes pretty much the same products as ordinary oxygen burning (T9 ≈ 2) at low η ≈0.002 (Z/Z ) Principal Products: 28Si, 32,33,34S, 35,37Cl, 36,38Ar 39,41K, 40,42Ca, 46Ti, 50Cr 4) Explosive neon burning (2.5≤T9 ≤3.0) Same products as stable hydrostatic neon buring More 26Al, the p-process or γ -process. 5) Explosive H and explosive He burning. The former occurs in novae; the latter in some varieties of Type Ia supernovae. Discuss later. The “p -” or - Process At temperatures ~2 x 109 K before the explosion (oxygen burning) or between 2 and 3.2 x 109 K during the explosion (explosive neon and oxygen burning) partial photodisintegration of pre-existing s-process seed makes the proton-rich elements above the iron group. The p-Process (aka the -process) p-nuclei A 25 M Supernova Problems below A ~ 130. Summary: Process • Makes nuclei traditionally attributed to the “p-process” by photodisintegration of pre-existing s-process nuclei. The abundance of these seeds is enhanced – at least for A < 90 – by the s-process that went on in He and C burning. • Partially produced in oxygen shell burning before the collapse of the iron core, but mostly made explosively in the neon and oxygen- rich shells that experience shock temperatures between 2 and 3.2 billion K. • Production factor ~100 in about 1 solar mass of ejecta. Enough to make solar abundances • A secondary (or tertiary) process. Yield is proportional to abundance of s-process in the star. • There remain problems in producing sufficient quantities of p-nuclei with atomic masses between about 90 and 120, especially 92Mo. The Neutrino Process (-process) The neutrino flux from neutron star formation in the center can induce nuclear transmutation in the overlying layers of ejecta. The reactions chiefly involve µ and -neutrinos and neutral current interactions. Notable products are 11B, 19F. 138La, 180Ta, and some 7Li and 26Al. Production factor relative to solar normalized to 16O production as a function of µ and τ neutrino temperature (neutral current) and using 4 MeV for the electron (anti-)neutrinos (for charged current only). Product 6 MeV 8 MeV 6 MeV 8 MeV This This WW95 WW95 This work WW95 This work WW95 work work 11B 1.65 1.88 3.26 3.99 0.95 1.18 1.36 1.85 19F 0.83 0.60 1.28 0.80 0.56 0.32 1.03 0.53 15N 0.46 0.49 0.54 0.58 0.09 0.12 0.15 0.19 138La 0.97 1.10 0.90 1.03 180Ta 2.75 3.07 4.24 5.25 Heger et al,, 2005, Phys Lettr B, 606, 258 Integrated Ejecta Averaged yields of many supernovae integrated either over an IMF or a model for galactic chemical evolution. Survey - Solar metallicity: (Woosley and Heger 2007) • Composition – Lodders (2003); Asplund, Grevesse, & Sauval (2004) • 32 stars of mass 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 35, 40, 45, 50, 55, 60, 70, 80, 100, 120 solar masses. More to follow. • Evolved from main sequence through explosion with two choices of mass cut (S/NAkT = 4 and Fe-core) and two explosion energies (1.2 foe, 2.4 foe) – 128 supernova models • Averaged over Salpeter IMF Woosley and Heger, Physics Reports, 442, 269 - 283, (2007) Isotopic yields for 31 stars averaged over a Salpeter IMF, = -1.35 Intermediate mass elements (23< A < 60) and s-process (A = 60 – 90) well produced. Carbon and Oxygen over- produced. p-process deficient by a factor of ~4 for A > 130 and absent for A < 130 Survey Z = 0; 10 to 100 M (Heger & Woosley, 2010, ApJ, 724, 341) Big Bang initial composition, Fields (2002), 75% H, 25% He 10−12M ΔM = 0.1M 12−17 M ΔM = 0.2 M Evolved from main sequence to 17 - 19 M ΔM = 0.1 M presupernova and then exploded 19− 20 M ΔM = 0.2 M with pistons near the edge of the iron core (S/N k = 4.0) 20 - 35 M ΔM = 0.5 M A 35 - 50 M ΔM = 1 M 50 - 100 M ΔM = 5 M Each model exploded with a variety of energies from 0.3 to 126 Models 10 x 1051 erg. at least 1000 supernovae “Standard model”, 1.2 B, = 1.35, mix = 0.1, 10 - 100 solar masses Best fit, 0.9 B, = 1.35, mix = 0.0158, 10 - 100 solar masses Lai et al. 2008, ApJ, 681, 1524 28 metal poor stars in the Milky Way Galaxy -4 < [Fe/H] < -2; 13 are < -.26 Eexp Cr I and II, non-LTE effects; see also KE = Eo (20/M) B Sobeck et al (2007) mixing 0.1 would have been "normal" and now 17O Woosley, Heger, & Weaver (2002) The r-Process The rapid addition of neutrons to iron group nuclei that produces the most neutron-rich isotopes up to uranium and beyond. This is though to occur either in the deepest ejecta of supernovae or in merging neutron stars. The r-Process The r-Process The r-process path hits the closed neutron shells for a smaller value of A (i.e., a lower Z) These heavy nuclei cannot be made by the s-process, nor can they be made by charged particle capture or photodisintegration. Photodisintegration would destroy them and make p-nuclei. The temperatures required for charged particle capture would destroy them by photodisintegration. Their very existence is the proof of the addition of neutrons on a rapid, explosive time scale. This requires a high density of neutrons. They were once attributed to the Big Bang, but the density is far too low.
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